Electric resistance heating methods include induction heating and direct resistance heating. In hardening treatment in particular which is one kind of heat treatment using induction heating, a proper frequency is selected according to a heat treatment depth in a workpiece.
According to a related art power conversion apparatus for heat treatment, DC power is converted into high-frequency power by performing switching using a power semiconductor device. In recent years, an SiC-MOSFET made of SiC (silicon carbide) has attracted attention as a switching device for the power semiconductor.
The switching frequency becomes ten times when the oscillatory frequency changes from 10 kHz to 100 kHz. Thus, the temperature rise of the power semiconductor changes greatly. That is, when the capacity (maximum rated value) of an inverter in the power conversion apparatus for heat treatment is determined in accordance with the maximum frequency in an operating range of the apparatus, the temperature rise is uneconomically low for a low output frequency. It is also uneconomical that the temperature rise differs in accordance with a current-applying time per one time as well as in accordance with the operating frequency.